The subject matter of this disclosure relates generally to photovoltaic (PV) systems and more particularly to a system and method of integrating solar modules, inverters and frames into a common rack structure.
Nearly all electrical systems in the U.S. are grounded to mitigate the impacts of lightning, line surges, or unintentional contact with high voltage lines. Most PV systems include modules with metal frames and metal mounting racks that are in exposed locations, e.g. rooftops, where they are subject to lightning strikes, or are located near high voltage transmission lines that in the event of high winds, etc., can come into contact with PV arrays.
The modules in a typical PV array have aluminum frames that are often anodized. The 2008 National Electrical Code (NEC) that has the same requirements as the draft 2010-NEC and governs installation of PV systems requires exposed metal surfaces be grounded. There are special dc wiring and grounding requirements that must be met specifically for dc module strings that can produce voltages at high as 600 volts (V). A failure in the insulating material of the PV laminate could allow the frame to be energized up to 600 V dc.
The installer of a PV system is required to ground each module frame per the NEC and Underwriters Laboratories (UL) standard 1703. This inter-module grounding must be met using a heavy (e.g. at least #10 gauge) copper wire and a 10-32 screw that can cut into the frame. Additional assurances are required even for frames having anodized surfaces. Washer/connectors in such cases are used to cut into the metal frame and provide the best electrical contact. These processes require additional components for installation and require a substantial level of effort to install mounting brackets and grounding wires.
Grounding continuity must also be addressed per the NEC. The oldest NEC requirement necessitates making the ground connection first and breaking the ground connection last. Not all installations follow this practice. The NEC also indicates that the circuit conductors should never be connected without a solid ground in place. Ground fault interruption (GFI) cannot prevent shock in this situation.
Presently, commercial systems that employ micro-inverters still commonly require an equipment ground, meaning that all modules with metallic frames and metal mounting systems have to be connected to a common earth ground through a low resistance path. Such inter-module ground connections are made using processes that require the use of metallic splices, lugs, penetrating washers, and wires. All of these methods require that grounding connections be made at the time of installation and usually require the presence of an experienced electrician. The electrical component of PV module installation is now the largest single cost. Much of the cost associated with installation of PV modules is the expense of an on-site electrician.
AC modules have increasing importance in the solar power generation industry. DC to AC conversion that is local to each solar electric module has certain advantages for residential solar power generation systems. These advantages include without limitation, availability, high energy yield, simple interconnections, and the like. Most implementations of solar AC modules involve the interconnection of a microinverter with a solar electric module. The microinverter is mounted onto the rail and DC-DC cabling is used to implement the connection in some installations. The AC connections are made in parallel through output cables on the microinverters. The microinverter is attached directly to the solar electric module in other installations by bolting the microinverter to the frame of the solar electric module; and the electrical interconnections are made in the same way. The foregoing installation techniques result in no physical change to the solar electric module system that generally requires a junction box to house the DC wiring; and bypass diodes remain in place. Up to ⅓ the cost of a microinverter includes the metal case/heatsink, wiring and connectors.
In view of the foregoing, it would be advantageous to provide a system and method for integrating solar modules, inverters and supporting frames in a manner that reduces installed costs through the integration of certain mechanical and electrical functions normally associated with individual components of the system. The system and method should provide a simple means for a roofing contractor responsible for the mechanical installation to complete the electrical connection between the modules without requiring the presence of an electrician.